(1) Field of the Invention
The subject-matter of the invention is a method to level a flat product in strip or sheet form in a levelling machine with intermeshed rolls, and the levelling installation which can be used to implement the method.
(2) Prior Art
To level flat products and, in particular rolled metal strips and sheet, a levelling machine is often used having multiple rolls comprising two sets of levelling equipment each carrying a series of rolls with parallel axes and respectively positioned above and below the strip, the rolls being staggered longitudinally and vertically so that they intermesh thereby defining a bending pathway for the work strip which is therefore subjected to tensile bending effects in alternate directions. Motors are used to actuate the rolls in rotation and, by friction, the product is caused to travel forward at a determined velocity. To level thick strips and sheet metal these sets of equipment operate without the application of any external tension forces either upstream or downstream of the machine.
The functioning of said machine has been the subject of advanced theoretical approaches. These levelling theories are based on the calculation of the maximum curvatures of the sheet metal in the levelling machine, these curvatures generating plastic deformation of the material in the thickness of the product which determines the relaxing of stresses in width and thickness. Depending on the extent of roll intermesh and diameter, the product assumes a greater or lesser curvature, and plastic deformation affects a greater or smaller part of the thickness. One adjustment parameter for levelling is the plastic deformation rate, which represents the ratio between the thickness of the product in which the stress has exceeded the elastic limit, deformation is therefore plastic, compared with the total thickness of the product. Evidently the remainder of the thickness of the product remains at an elastic deformation stress value. These two regions in the thickness of the product each have an influence on levelling force and the torque values to be transmitted to the rolls, but the variations they generate with respect to levelling parameters follow different laws.
Finally, in a levelling machine a distinction is generally made between two zones, whose functions are substantially different but complementary and interactive. An entry zone which is a zone of substantial plastic deformation preferably treating geometric planarity defects, and an output zone in which levelling more concerns de-tensioning and takes place with low plastic deformation or even by alternate bending within the elastic limit of the material.
It is known how to establish theoretical calculations to arrive at an accurate result and which may be used for pre-adjustment of the machines. The principle is based on progressive decrease of the plastic deformation rate. This can only be achieved with machines comprising a sufficient number of levelling rolls and allowing adequate adjustment of the intermesh of each of its rolls.
Roll intermesh depends on the required plastic deformation rate and on the thickness and temperature of the sheet to be levelled. For steel sheet leaving a hot rolling mill, generally a high plastic deformation rate is required, in the order of 70%. To obtain this rate with thick sheet only a slight intermesh is needed, but at the same time levelling forces are very high having a tendency to draw the levelling rolls away from one another and to reduce the intermesh. This means that the deflection of the rolls, also called camber, produced by the levelling forces is much higher than the value of intermesh needed for plastic deformation. A problem is therefore raised relating to the precision with which it is possible to control the value of the intermesh, and of determining a control method to ensure the same.
A levelling installation therefore generally comprises a fixed support stand, two sets of levelling equipment with parallel rolls, respectively positioned above and below the workpiece and whose rolls intermesh so as to define a bending pathway for the workpiece, and means to adjust and maintain the spacing of said sets by action on the fixed stand to adjust the intermeshing of the rolls, each set of levelling equipment comprising a row of parallel work rolls bearing on a support frame via at least one row of back-up rolls, each mounted rotatably at their ends on two bearings defining a rotation axis perpendicular to the direction of travel, said bearings being respectively carried by two lateral parts secured to the supporting frame.
In a levelling machine to level very thick sheet and strip metal, the work rolls are motorized by electric motors since substantial torque must be transmitted to each roll to ensure deformation and forward travel of the product inside the levelling machine.
Most often the lower levelling set is fixed in position, the upper levelling set being able to move vertically for adjustment of the intermesh. For this purpose four mechanical or hydraulic actuators are generally used, mounted at the corners of the frame and allowing the general level of the adjustable set of equipment to be adjusted relative to the fixed lower set, and hence providing adjustment of the roll intermesh. Additionally, the actuators can be adjusted independently and at different values, making it possible to determine switching between the entry and exit of the levelling machine, generally needed when it is required to achieve different levelling effects according to needs.
Efforts developed for levelling are very high, in particular when this operation is conducted on strong sheet metal after hot rolling and accelerated cooling, or on cold sheet. This leads to providing levelling machines with a structure that is as rigid as possible, so that levelling effects can be controlled. Deformation of the different parts of the machine under load distorts the accuracy of control over the position of the levelling rolls, and hence of the curvature value obtained on each roll, and may in some cases make such accuracy impossible.
For example, for steel sheet 50 millimeters thick, the calculated value of the intermesh is in the order of 0.3 millimeter, however the levelling forces are sufficiently high to cause camber of the machine in the order of a dozen millimeters. It is therefore obvious that the direct application of a levelling method such as the one described in U.S. Pat. No. 4,881,392 is not possible. Also, rolling tolerance on sheet metal such as cited is in the order of 0.1 millimeter. However a levelling machine must be able to open up to the value of the nominal thickness of the sheet metal to allow it to pass, but variations in thickness due to manufacturing tolerances must not cause variations in the plastic deformation rate, and must not therefore cause a variation in roll intermesh.
To overcome these drawbacks, levelling machines with hydraulic control have been produced for displacement of the mobile set of levelling rolls, and also for individual control of each levelling roll, and a camber model based on force measurements has been associated with a theoretical calculation of machine deformation under load, to offset these deformations, but all such offsets are flawed due to the non-linearity of machine camber and it is not realistic to consider offsetting a defect whose amplitude is ten or more times greater than the parameter to be controlled.